1.
Add formatted text to WPF RichTextBoxAdv in UI:
We can directly add text to WPF RichTextBoxAdv control in UI, by placing the
cursor at corresponding position and enter text. To format, select the inserted
text and apply required formatting. For further reference regarding the WPF RichTextBoxAdv control, please do
refer UG documentation from below link:
http://help.syncfusion.com/ug/wpf/tools/default.htm#!documents/richtextboxadvcontrol.htm

2.
Add formatted text to WPF RichTextBoxAdv in code behind:
We have prepared the sample to add formatted text at cursor position in WPF
RichTextBoxAdv control. Please try running the attached sample and let us know
if this helps you.
Please let us know if you have any other questions.